This invention relates to quantum well devices and to a method of changing and/or controlling the effective bandgap energy in quantum well structures, particularly Indium Gallium Arsenide Phosphide (InGaAsP) devices or structures. More particularly, it is concerned with enabling the integration of multiple optoelectronic devices within a single structure, each comprising a quantum well structure.
The semiconductor industry is currently interested in integrating various optoelectronic devices, such as lasers, modulators and detectors within a single semiconductor structure. This initiative is motivated by the increasing demand for optoelectronic technology particularly in the optical telecommunications field.
Integrated optoelectronic devices are of great interest due to the optical alignment and optical coupling efficiency challenges associated with using discrete optoelectronic devices. Within an integrated optoelectronic device, each optical component is spatially self aligned as result of being fabricated within the same semiconductor structure. This inherently gives better transmission between the components of an integrated device, as compared to putting together discrete devices. However, in order to ensure that the separate optical components within the structure each has its own independent characteristics, local modifications to the semiconductor quantum well structure of each component are usually necessary. Many known fabrication techniques for one component of an integrated structure tend to have the unwanted effect of distorting or affecting properties of neighbouring components.
Quantum Well Intermixing (QWI) is a Post-growth method of bandgap engineering known in the art, enabling controlled changes in the bandgap energy of selected regions of the quantum well structure. Quantum Well Intermixing uses a Rapid Thermal Annealing (RTA) process also known in the art, to provide controlled diffusion of defects into the quantum well structure of an optoelectronic device. These defects are usually provided by a layer or layers of specially grown material that are grown above the quantum well structure. Under the influence of the RTA process, the defects diffuse down into the quantum well structure and introduce changes to the bandgap properties. QWI has attracted considerable interest in locally modifying the quantum well band structure of integrated optoelectronic devices, including tunable wavelength lasers, photodetectors, and modulators. It is believed to be capable of modifying one component with minimum impact on neighbouring components.
Different thermally-driven quantum well intermixing techniques such as Ion-Implantation Disordering (IID), Impurity Free Defect Diffusion (IFDD), Photo-absorption Induced Disordering (PAID) and Impurity-Induced Layer Disordering (IILD) have been utilized in order to modify the quantum well structure in selected regions.
In Ion Implantation Disordering (IID), high energy implanted ions may introduce lattice damage to the quantum well structure, resulting in reduced light output. The Impurity-Induced Layer Disordering (IILD) technique requires long anneal times and/or high anneal temperatures ( greater than 800xc2x0 C.) for diffusing impurities into the quantum well region. This can cause undesirable changes in the characteristics of neighbouring components within an integrated optoelectronic devices. It also introduces unwanted impurities, causing undesirable changes to the properties of the quantum well structure. The Impurity Free Defect Diffusion (IFDD) technique is free of impurities, but control of the QWI process depends on the deposited cap layer being used, it""s deposition conditions and the subsequent thermal anneal treatment. If for example, a silicon dioxide (SiO2) cap layer is used, the thermal anneal process requires the use of temperatures between 750-800xc2x0 C. These anneal temperatures may cause an uncontrollable shift in device operating wavelength, such as, the emission wavelength of laser devices. Also, the surface of the grown material may become unstable and therefore unsuitable for subsequent processing of components such as gratings. Furthermore, strain and damage may be introduced to the hetero-structure surface. Finally, Photo-absorption Induced Disordering (PAID) suffers from poor spatial resolution. Consequently, it is difficult to confine this effect to an intended component within an integrated device, without affecting adjacent components.
Accordingly, there is need for a QWI process wherein, surface contamination, uniformity and strain effects are avoided, and which is reproducible. With these characteristics, it should be possible to import desired properties to one component of an integrated structure, without affecting neighbouring components. Furthermore, there is a need for a QWI process that enables the high speed operation of optoelectronic devices, and affects the photo-luminescent properties of the quantum well structure.
The present invention discloses a Quantum Well Intermixing (QWI) method for locally modifying the effective bandgap energy in Indium Gallium Arsenide Phosphide (InGaAsP) quantum well structures. This quantum well intermixing method involves growth of a first Indium Phosphide layer with slow diffusing defects grown near the upper quaternary layers of the quantum well structure at normal temperature using Gas Source Molecular Beam Epitaxy (GSMBE). A second Indium Phosphide layer with fast diffusing defects is also grown near the surface of the quantum well structure at normal temperature using Gas Source Molecular Beam Epitaxy (GSMBE). By applying a rapid thermal annealing (RTA) process, both the slow diffusing defects from the first Indium Phosphide layer, and fast diffusing defects from the second Indium Phosphide layer, diffuse to the quantum well active region. This controlled inter-diffusion process provides localised, controlled changes in the properties and bandgap energy of the quantum well active region.
An alternative embodiment to the present invention includes a quantum well intermixing process, wherein an Indium phosphide layer with point defects is grown near the surface of the quantum well structure at low temperature using Gas Source Molecular Beam Epitaxy. By applying rapid thermal annealing (RTA), the point defects in the Indium Phosphide layer diffuse to the quantum well region. This controlled inter-diffusion process provides an increasingly high effect on the effective bandgap energy of the quantum well active region.
In another aspect of the present invention, quantum well intermixing is used in order to modify the effective bandgap properties of an integrated optoelectronic device comprising a laser and electro-absorption modulator. The quantum well intermixing process is applied to the electro-absorption modulator region of the integrated optoelectronic device. Following the quantum well intermixing process, the effective bandgap properties of the Indium Gallium Arsenide Phosphide (InGaAsP) quantum well active region of the modulator are modified. These modifications introduce both an ultra-fast response, and increased efficiency in the operating characteristics of the electro-absorption modulator. This enables the integration of high speed quantum well devices with standard quantum well devices.